MADISON, Wis. (AP) -- Potential Democratic candidates who may challenge Republican Gov. Scott Walker in a recall election next year are scheduled to gather at a rally Tuesday in Madison.

Former congressman Dave Obey, state Sen. Jon Erpenbach and former Dane County Executive Kathleen Falk are also scheduled to take part in a tour organized by the Democratic Party to highlight its plans to attempt a Walker recall.

The petition drive to force the recall is set to begin Nov. 15. More than 540,000 signatures would have to be collected by Jan. 17 to force an election sometime later in 2012.

The rally is the first in a series this week that will also serve as training sessions for volunteers circulating petitions. Other stops include Stevens Points, Eau Claire, Appleton and Milwaukee.